Hydraulic vehicular braking systems are well known to engineers of ordinary skill in the art and are incorporated in virtually all of the vehicles currently in service. The two most common types of brake systems now in use in cars are hydraulic drum brakes and hydraulic disc brakes. A hydraulic brake system operates by using pressurized fluid from a master cylinder to force a friction member against a rotating rotor or drum. The rotor or drum rotates with a wheel, while the friction member is non-rotating. A disc brake system incorporates a non-rotating brake pad capable of being hydraulically forced against the rotating rotor or rotating drum to apply brake torque to the wheel. Both drum and disc brake systems use a master hydraulic cylinder to translate a signal from the operator into a braking signal to the wheel hydraulic brake cylinders that push the friction members against the rotors to brake the wheels. The wheel hydraulic cylinders and corresponding friction members are positioned at each of the wheels to be braked.
When the operator desires to slow or stop a car with a hydraulic brake system, he engages a control actuator. The control actuator is commonly a foot pedal in fluid communication with the master cylinder. Depression of the foot pedal pressurizes the master cylinder. Pressurization of the master cylinder results in pressurized fluid being sent to the individual brake cylinders connected to each wheel. Pressurization of an individual brake cylinder forces a brake-operating member (such as piston coupled to a brake pad) against a portion of the wheel assembly. Intermediary pumps help to maintain and amplify the hydraulic pressure to brake cylinders, so that sufficient force is applied to the brake discs or brake drums to slow or stop the car.
The frictional force provided by the brake to slow the wheel is affected by the frictional forces acting between the wheel and the road. When the friction between a given wheel and the road is relatively small, for example when the road is slick, the brake friction will have a relatively great effect on that wheel. In that event, the applied braking force may excessively slow or even stop its rotation relative to the other wheels (in contact with a relatively normal road surface). This can result in a skid event and a corresponding loss of vehicle control. Anti-lock brake systems have been developed that automatically incrementally reduce brake cylinder pressure. These systems indirectly measure the resultant torque on the wheel from the forward momentum of the vehicle and the applied braking friction and, if too great, reduce braking pressure until the braking torque drops below a predetermined threshold point corresponding to the resumption of regular wheel rotation. Pressure is then allowed to rebuild. If the torque again reaches the critical point, pressure is again relieved.
Recently an electrically controlled brake system has been proposed wherein the required operating force or stroke of the brake-operating member is electrically determined. The amount of brake force applied to the wheel would be controlled so as to provide a braking effect that corresponds to the required amount of brake force determined by the operating member. Examples of such an electrically controlled brake system are disclosed in U.S. Pat. No. 5,333,944 to Shirai. That reference discloses a system wherein the hydraulic pressure in the wheel brake cylinder is electrically controlled to provide a suitable deceleration value of the vehicle.
In the Shirai system, the deceleration value is governed by the electrically detected operating force determined by the brake-operating member. The hydraulic pressure in an accumulator is controlled by a solenoid-operated pressure control valve, and the controlled hydraulic pressure is applied to the wheel brake cylinder, to force a brake pad against the rotor or drum so that the wheel rotating with the rotor or drum is braked. The solenoid coil current is determined by the controller so that the hydraulic pressure applied to the wheel cylinder is such that the detected actual deceleration value of the vehicle matches the target or desired deceleration value determined based on the electrically detected operating force determined by the brake operating member. The amount of pressure applied to the friction member is controlled such that the detected amount of output force coincides with the target reaction force determined by the brake-operating member.
Prior systems, such as the Shirai system, have relied on pressurized hydraulic fluid flowing from a central source, such as a master cylinder or accumulator, through valves into an individual wheel cylinder to exert pressure on a friction member such as a brake pad against the rotor or wheel drum to brake the vehicle. Such systems suffer from a number of disadvantages. One disadvantage is the requirement of the vehicle's motor to be running in order to maintain a sufficient available hydraulic pressure. Another disadvantage is that the braking of all wheels relies on the central fluid source. Damage to the central fluid source causing depressurization or obstructing the flow of the hydraulic fluid can disable braking of all four wheels. Still another disadvantage is the extra weight and volume requirements of the master cylinder. Yet another disadvantage is the expense of the hydraulic components and fixing or replacing them. Prior systems have also required the controlled opening and closing of a hydraulic valve to apply and remove the braking torque from the wheels. Moreover, there is a finite lag in the response time between the operator's actuation of the brake controls and the application of the full braking torque to the wheels arising from the finite time required for the solenoid to open the valves and the fluid to achieve full pressure against the friction member.
Consequently, there is a need for a faster, less expensive, lighter, and more efficient vehicular braking system that among other things does not require the motor to be running in order to actuate the braking system and wherein braking is not contingent upon the integrity of a single fluid source. The present invention meets this need.